Most current research into the causes of cancer focuses on genes and environmental triggers. Evolutionary biologist Paul Ewald of the University of Louisville in Kentucky argues that scientists have overlooked the most important cause: parasites, especially viruses. Blending medicine and Darwinian biology, Ewald considers cancer and other diseases from the pathogen’s point of view, showing how natural selection determines why the smallpox virus, for instance, is a ruthless killer while viruses for the common cold are relatively benign. He says that once we identify the viruses that trigger cancer, we can work to prevent their transmission and force them to evolve from fatal scourges into mere nuisances, eventually turning cancer into a manageable disease.